1. Field of the Invention
This invention relates to the manufacture of synthetic fuels from coal and similar carbonaceous solids and is particularly concerned with integrated coal liquefaction processes in which the coal liquids produced by liquefaction are subsequently hydrogenated.
2. Description of the Prior Art
Processes for the liquefaction of coal and similar carbonaceous solids normally require contacting of the solid feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure to break down the complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. The most promising processes of this type are those carried out with a hydrogen-donor solvent which gives up hydrogen atoms for reaction with organic radicals liberated from coal or other feed material during the liquefaction step. In such processes, the hydrogen-donor solvent is subsequently regenerated in a downstream solvent hydrogenation step. Plants for the carrying out of processes of this type normally include facilities for generation of the needed molecular hydrogen by the gasification of heavy liquefaction bottoms produced in the liquefaction step, by the coking of liquefaction bottoms and subsequent gasification of the resultant coke, by the reforming of light hydrocarbon liquids and gases produced in the process, or by other means.
The coal and other solid feedstocks employed in liquefaction operations normally include substantial quantities of oxygen which participates in the reactions taking place during liquefaction. This results in the formation of oxygenated organic compounds such as acids, phenols, cresols, dihydroxy-substituted benzenes, and the like. The specific compounds produced and the quantities in which they are present in the liquid and gaseous product streams depend in part upon the composition of the feed material and the liquefaction conditions employed. Some of these oxygenated compounds are normally concentrated in waste water formed by the cooling of liquid and vapor streams and must be removed from the waste water before it is reused or discharged. Other oxygenated compounds are carried over with the intermediate boiling range liquefaction products to the solvent hydrogenation zone where they are in part converted to the corresponding hydrocarbons during the solvent hydrogenation step. This results in the consumption of considerably more hydrogen than would otherwise be required for regeneration of the solvent and thus adds significantly to the cost of the overall process. Methods for the removal of residual oxygenated materials from the product streams further increase process costs.